In an optical head device for performing recording or reproduction of information into or from an optical recording disk such as a CD or a DVD, it has been conventionally known that the performance of recording and reproduction is deteriorated by the affection of the wave front aberration which is generated due to various factors. An aberration compensating device for compensating the wave front aberration has been proposed, which includes a liquid crystal panel having a liquid crystal layer whose refractive index is changed in accordance with an applied voltage and transparent electrodes disposed so as to interpose the liquid crystal layer therebetween and a liquid crystal panel control part controlling a voltage applied to the transparent electrode. The aberration compensating device compensates the aberration of a laser beam which is emitted from a light source and converged on an optical recording disk through an objective lens.
As an example, an aberration compensating device has been known in which the transparent electrode comprises a center part in an approximately circular shape and outer peripheral divided parts which are divided into eight sections at an approximately equal angle around the center part and disposed so as to surround the center part (see, for example, Japanese Patent Laid-Open No. 2000-40249). In the aberration compensating device, voltages applied to divided areas comprising of the center part and the respective outer peripheral divided parts are controlled corresponding to the direction of the astigmatism of a laser beam passing through the liquid crystal panel. The astigmatism is compensated by applying a phase difference corresponding to the voltage, which is controlled in the respective divided areas, to the laser beam passing through the respective divided areas.
In the aberration compensating device described in Japanese Patent Laid-Open No. 2000-40249, even when astigmatism is generated in various directions in an optical system used in an optical head device, the transparent electrode is provided with the divided areas comprising of the center part and the respective outer peripheral divided parts so as to cope with the direction of the astigmatism, and the phase difference corresponding to the voltage applied to the respective divided areas is applied to the laser beam passing through the respective divided areas. Therefore, the astigmatism whose direction or magnitude is different can be effectively compensated with a simple structure.
However, the compensation of the astigmatism by the conventional aberration compensating device described above is not sufficient, and thus an aberration compensating device having a further enhanced aberration compensating effect is required to ensure the recording and reproduction performance of an optical head device.